1. Field of the Invention
The present invention provides a process for obtaining one or more than one organic salt or organic acid from an aqueous sugar stream. More particularly, the invention relates to a process for obtaining one or more than one organic salt or organic acid from an aqueous sugar stream comprising one or more than one mineral acid and a sugar(s).
2. Related Art
Fuel ethanol is currently produced from feedstocks such as corn starch, sugar cane, and sugar beets. However, the potential for production of ethanol from these sources is limited as most of the farmland which is suitable for the production of these crops is already in use as a food source for humans. Furthermore, the production of ethanol from these feedstocks has a negative impact on the environment because fossil fuels used in the conversion process produce carbon dioxide and other byproducts.
The production of ethanol from cellulose-containing feedstocks, such as agricultural wastes, grasses, and forestry wastes, has received much attention in recent years. The reasons for this are that these feedstocks are widely available and inexpensive and their use for ethanol production provides an alternative to burning or landfilling lignocellulosic waste materials. Moreover, a byproduct of cellulose conversion, lignin, can be used as a fuel to power the process instead of fossil fuels. Several studies have concluded that, when the entire production and consumption cycle is taken into account, the use of ethanol produced from cellulose generates close to nil greenhouse gases.
The lignocellulosic feedstocks that are the most promising for ethanol production include (1) agricultural wastes such as corn stover, wheat straw, barley straw, oat straw, rice straw, canola straw, and soybean stover; (2) grasses such as switch grass, miscanthus, cord grass, and reed canary grass; and (3) forestry wastes such as aspen wood and sawdust.
The three primary constituents of lignocellulosic feedstocks are cellulose, which comprises 30% to 50% of most of the key feedstocks; hemicellulose, which comprises 15% to 35% of most feedstocks, and lignin, which comprises 15% to 30% of most feedstocks. Cellulose and hemicellulose are comprised primarily of carbohydrates and are the source of sugars that can potentially be fermented to ethanol. Lignin is a phenylpropane lattice that is not converted to ethanol.
Cellulose is a polymer of glucose with beta-1,4 linkages and this structure is common among the feedstocks of interest. Hemicellulose has a more complex structure that varies among the feedstocks. For the feedstocks of interest, the hemicellulose typically consists of a backbone polymer of xylose with beta-1,4 linkages, with side chains of 1 to 5 arabinose units with alpha-1,3 linkages, or acetyl moieties, or other organic acid moieties such as glucuronyl groups.
The first process step for converting lignocellulosic feedstock to ethanol involves breaking down the fibrous material. The two primary processes are acid hydrolysis, which involves the hydrolysis of the feedstock using a single step of acid treatment, and enzymatic hydrolysis, which involves an acid pretreatment followed by hydrolysis with cellulase enzymes.
In the acid hydrolysis process, the feedstock is subjected to steam and a mineral acid, such as sulfuric acid, hydrochloric acid, or phosphoric acid. The temperature, acid concentration and duration of the hydrolysis are sufficient to hydrolyze the cellulose and hemicellulose to their monomeric constituents, which is glucose from cellulose and xylose, galactose, mannose, arabinose, acetic acid, galacturonic acid, and glucuronic acid from hemicellulose. Sulfuric acid is the most common mineral acid for this process. The sulfuric acid can be concentrated (25-80% w/w) or dilute (3-8% w/w). The resulting aqueous slurry contains unhydrolyzed fiber that is primarily lignin, and an aqueous solution of glucose, xylose, organic acids, including primarily acetic acid, but also glucuronic acid, formic acid, lactic acid and galacturonic acid, and the mineral acid. The aqueous solution is separated from the fiber solids to produce a sugar hydrolyzate stream.
In the enzymatic hydrolysis process, the steam temperature, mineral acid (typically sulfuric acid) concentration and treatment time of the acid pretreatment step are chosen to be milder than that in the acid hydrolysis process. Similar to the acid hydrolysis process, the hemicellulose is hydrolyzed to xylose, galactose, mannose, arabinose, acetic acid, glucuronic acid, formic acid and galacturonic acid. However, the milder pretreatment does not hydrolyze a large portion of the cellulose, but rather increases the cellulose surface area as the fibrous feedstock is converted to a muddy texture. The pretreated cellulose is then hydrolyzed to glucose in a subsequent step that uses cellulase enzymes. Prior to the addition of enzyme, the pH of the acidic feedstock is adjusted to a value that is suitable for the enzymatic hydrolysis reaction. Typically, this involves the addition of alkali to a pH of between about 4 and about 6, which is the optimal pH range for cellulases, although the pH can be higher if alkalophilic cellulases are used.
In one type of pretreatment process, the pressure produced by the steam is brought down rapidly with explosive decompression, which is known as steam explosion. Foody, (U.S. Pat. No. 4,461,648) describes the equipment and conditions used in steam explosion pretreatment. Steam explosion with sulfuric acid added at a pH of 0.4 to 2.0 has been the standard pretreatment process for two decades. It produces pretreated material that is uniform and requires less cellulase enzyme to hydrolyze cellulose than other pretreatment processes.
Regardless of whether acid hydrolysis or enzymatic hydrolysis is carried out, the resulting aqueous hydrolyzate stream is likely to contain glucose, xylose, arabinose, galactose, mannose, and organic acids, such as acetic acid, glucuronic acid, formic acid and galacturonic acid and the mineral acid, such as sulfuric acid. However, it will be appreciated that salts of the mineral acid and organic acid may be present and that the fraction of these acids in the salt form will increase with increasing pH. The glucose in this stream can be readily fermented to ethanol by conventional yeast or to butanol by bacteria. The pentose sugars can be fermented to ethanol by recombinant yeast (see U.S. Pat. No. 5,789,210 (Ho et al.) and WO 03/095627 (Boles and Becker)) or bacteria. Alternatively, the pentose sugars may be used as starting materials for the generation of other high value products using chemical, microbial or enzymatic means or simply recovered. For example, xylitol may be produced by the fermentation or hydrogenation of xylose or the xylose may be simply recovered.
The presence of the organic acid and mineral acid, or the corresponding salts, in a hydrolyzate stream decrease the efficiency of processes for converting glucose or other sugars to ethanol or other valuable products. In particular, during any neutralization conducted prior to enzymatic hydrolysis or fermentation (both of which take place at moderate pH values such as at pH values of about 4.0 to about 6.0), these compounds will consume alkali, such as sodium hydroxide, ammonium hydroxide, or potassium hydroxide. In addition, the mineral acids and organic acids, and their salts, may be inhibitory to yeast, bacteria and, to a lesser extent, cellulase enzymes. Any such inhibition can decrease the efficiency of the fermentation and enzymatic hydrolysis operations by lengthening the time required for carrying out the fermentation or enzyme hydrolysis, increasing the amount of yeast or enzyme catalyst required and/or decreasing the final yields. It therefore may be desirable to remove these compounds from the hydrolyzate to produce a clean sugar stream. In addition it may also be advantageous to remove these compounds from sugar streams obtained from other than hydrolysis, depending on the circumstances.
Pfeiffer (U.S. Pat. No. 4,102,705) discloses the deacidification of xylose streams by the removal of acetic acid and the mineral acids of sulfuric, hydrochloric, or nitric acid by using a two-stage ion exchange process. Pfeiffer feeds the aqueous stream to the first anion exchange system to bind the mineral acid and allow xylose and acetic acid to pass through. The resin is regenerated with sodium hydroxide, thereby producing sodium chloride, sodium sulfate, or sodium nitrate salt. The stream containing xylose and acetic acid is evaporated to remove 90% of the acetic acid. The resulting xylose stream with the remaining acetic acid is fed to a second ion exchange system, which binds the acetic acid and allows the deacidified xylose stream to pass through. The ion exchange resin is regenerated with sodium hydroxide to generate sodium acetate salt.
The evaporation taught by Pfeiffer would be very extensive in order to remove 90% of the acetic acid from the aqueous stream. Acetic acid is less volatile than water, so this evaporation would dewater the stream almost to dryness. It is very difficult to carry out such an evaporation as the presence of precipitated solids leads to scale deposition and fouling of heat exchange surfaces.
Wooley et al. (In Lignocellulosic Biomass to Ethanol Process Design and Economics Utilizing Co-Current Dilute Acid Prehydrolysis and Enzyme Hydrolysis Current and Future Scenarios, (1999) Technical Report, National Renewable Energy Laboratory pp. 16-17), reports removing 88% of the acetic acid and 100% of the sulfuric acid from a sugar hydrolyzate stream by using a continuous ion exchange separations unit. The ion exchange media is a weak base anion exchange resin and the resin is regenerated with ammonia. The acetic acid and sulfuric acid are discharged from the unit in the same stream and disposed of in a wastewater treatment unit.
WO 2006/007691 (Foody and Tolan) discloses the use of ion exclusion chromatography at pH 5.0 to 10.0 to separate ammonium acetate and ammonium sulfate salts from sugar streams prior to fermentation of the sugar. This separation method relies on the use of a cation exchange resin in the ammonium form.
Wooley et al., (Ind. Eng. Chem. Res., 1998, 37:3699-3709) discloses the use of ion exclusion chromatography with cation exchange resins in the hydronium form to separate acetic acid and sulfuric acid from sugar hydrolyzate streams. In this process, sulfuric acid is excluded from the resin and passes through the resin first while the non-ionic sugars move more slowly through the resin. The feed streams are at pH 3.0 and below, and the resulting process separates the stream into sulfuric acid, sugar, and acetic acid streams. However, control and recovery of the three product streams in the process would be difficult and costly.
Anderson et al. (Ind. Eng. Chem., 1955, 47:1620-1623) discloses the use of strong base anion exchange resins as a means of separating a strong mineral acid from water soluble organic material. In this process, the strong base anion exchange resin is first converted to the sulfate form. The mineral acid is retained by the resin bed and the water soluble organic material passes through the resin bed and is not bound. The method is useful for binding and recovering strong acids such as sulfuric and hydrochloric acids and relies on the absence of a significant interaction between the water soluble organic material and the resin bed. As long as the sulfate form of the resin is available, the mineral acid will bind the resin. However, the process does not result in the separation of an organic acid or its salt from an aqueous sugar stream.
Barrier et al. (Integrated Fuel Alcohol Production Systems for Agricultural Feedstocks, Phase III, Quarterly Technical Report for the Period April-June 1995. Submitted by Tennessee Valley Authority Office of Agricultural and Chemical Development, TVA Contract No. TV-540881, 1985) discloses the use of anion exchange resins, including weak base anion exchange resins, to recover sulfuric acid from a hydrolyzate stream. The method is useful for the recovery of sulfuric acid but results in a mixed sugar-organic acid stream. The mixed sugar-organic acid stream is sent directly to a yeast fermentation to produce ethanol. Caustic is added to adjust the fermentation pH and yeast media components are also added. The ethanol containing solution is subsequently distilled to produce a fuel ethanol. However, there is no disclosure of recovery of the organic acid which is understood to remain in the still bottoms after ethanol distillation and is not recovered.
Therefore, there is not a satisfactory process for recovering organic acids, or their corresponding salts, from aqueous sugar streams. The ability to remove organic acids, or their salts, from sugar streams remains a critical requirement to improve the efficiency of converting sugar to ethanol or other valuable products.